This will be pretty long. I've picked up a couple more from my original four.
This is a pretty long post, read on with caution. =P
Now, I haven't delved into Smash as much as others, so you'll probably think/know I'm wrong, but this is just what I've come to find from playing. ^_^
1. Metaknight.
When I got Brawl right when it came out, MetaKnight was one of the first characters I tried out. I really like using it because I find that it matched how I played Fox in Melee, aerial wise, where I go at them and do Bair, and in MK's case, Fair too. Though, Fox's air game in Melee was incredibly faster. Also the dash attacks are similar too. Same with the grabs, they both have grabbing similarities, as you can run up to them full speed and grab em into a down throw and then shine them or U-Smash (Fox). For MK, into a Fair for me. Also, since I've stopped using Jigglypuff in this, it's interception skill rivals that of the Puffmeister.
2. Mr.Game&Watch.
I picked him up during Melee, and I loved G&W from then on. Nice aerials, awesome ground game, D-tilt ftw (Melee). And with Brawl, while some moves were somewhat nerfed (D-tilt), He's still awesome.
p.s. I suppose it's kind of obvious they would keep him, but when some friends thought they would get rid of him or rumors were about, I was staunch firm that he'd return.
3. Wolf.
I like this guy. Pretty good speed, powerful moves, and like the Fox of Melee, he got a awesome Bair. Dash is pretty nice. I love the tilts, and the A combo, not to mention the Smash attacks. Only down fall I'd say, is the recoveries. If your off stage and not right below the edge, and you do Up-B directly above, you're go passed the edge, but you won't grab the ledge. So you have to be careful with that, and have good aim with it. Plus a good projectile attack. I always try to use at least one character that has a projectile. Though, that's not really the reason behind me choosing Wolf.
4. Diddy Kong.
I tried him out because I liked the D.K. games, but I never really cared much for D.K. and when I tried Diddy, I really liked the play style. I did neglect playing as him for a while though I still used him, just not competitively much, I became more attached to using him. I was playing my brother, and I did a up-throw and I managed it into a semi-wall of pain with three Bairs. That, and a killer Fair when timed right, and a pretty stellar Dair and Uair. Not to mention the Bananas, toss one at them and you can get a free F-Smash. >=O Forward-B is pretty cool too.
5. Wario.
This guy is just fun to use. Awesome aerial direct ability, pretty decent Bair, Fair is okay, and the Nair is decent too. Same with the Dair, decent. His Uair, is killer though. Then that Super armor (if that's what it's called) on the F-Smash. His D-Smash is long, but if it hits, they can't usually hit you. Tilts are pretty good. So is his A combo. I like down throwing into a F-Tilt or a F-Smash, though mostly the tilt. The Bike is an awesome recovery tool, it practically always allows you to get back on the stage. The bite move is hilarious, and pretty good at that. And don't get me started on the FART! Nuff said, AMIRITE?
6. Falco.
I never payed much attention to Falco in Melee, and likewise with Brawl. And I have to admit, when my friend owned me with it, I was curious to see it's full extent of it's ability, and I love the play style. And no, the Chain grab to Dair isn't the sole reason. =P Falco pretty much as the Bair that Fox lost, and overall good aerials. Nice F-Smash, D-Smash and like wise with the others. The A combo is decent. Plus a good projectile attack, and likewise with Wolf, not my main reason that I wanted to use him. To me, the Chain grab + Laser is just an added bonus for using him, not a sole reason for playing as him.
7. Bowser.
I always wanted to really play Bowser in Melee, and with the upgrades in Brawl, he's a lot better it seems like. I like his air game, Uair is kind of like D.K.'s, Fair is pretty similar to Wolf's, and the Bair is pretty good (Though, it should have higher priority, and as it finishes should make the foe flinch if they hit it). F-Smash have somewhat improved range, D-Smash is pretty good. If you land his U-Smash, that's a dangerous one. F-Tilt it pretty nice. I haven't really used the D-Tilt much though. U-tilt is good from what I've seen, but I accidentally keep turning it into a U-Smash because I press up and A too soon.
Those are who I main, and for 5-7, who I plan to also include in that list.